What is the first term for an arithmetic sequence with a12 = 56 and a21=101?

The first term is -43.

Explanation:

an = a + (n - 1)d

a12 = a + 11d = 56, first equation
a21 = a + 20d = 101, second equation

Subtract the second equation from the first equation, cancelling +a and -a
11d - 20d = 56 - 101
-9d = -45
d = 5

Subtitute d into the first equation
a + 99 = 56
a = -43

a refers to the first term, hence the first term is -43.